nixos/tests: Fix systemd-shutdown

Since 459ddaec261ae40149601e5bb487bd52936b4e50, stdout of PID 1
and children such as systemd generators and systemd-shutdown scripts
goes to `tty0` instead of `ttyS0`. This is because systemd's stdout is
`/dev/console`, which represents only one tty at a time, as explained
here: https://docs.kernel.org/admin-guide/serial-console.html

The reason PID 1's own output makes it to both consoles is because
it uses kmsg as its log target both before journald has started and
once the `systemd-shutdown` executable takes control. Unlike
`/dev/console`, kmsg output makes it to one console *of each type*,
i.e. one VT and one serial console.

0 files changed, 0 insertions, 0 deletions